Exact Flavor Dependence of the S-parameter

Preprint number: CP3-Origins-2010-32
Authors: Stefano Di Chiara (CP3-Origins), Claudio Pica (CP3-Origins), and Francesco Sannino (CP3-Origins)
External link: arXiv.org

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We extend the results of [arXiv:1006.0207 [hep-lat]] by computing the S-parameter at two loops in the perturbative region of the conformal window. Consistently using the expression for the location of the infra-red fixed point at the two-loop order we express the S-parameter in terms of the number of flavors, colors and matter representation. We show that S, normalized to the number of flavors, increases as we decrease the number of flavors and gives a direct measure of the anomalous dimension of the mass of the fermions. Our findings support the conjecture presented in [arXiv:1006.0207 [hep-lat]] according to which the normalized value of the S-parameter at the upper end of the conformal window constitutes the lower bound across the entire phase diagram for the given underlying asymptotically free gauge theory. We also show that the non-trivial dependence on the number of flavors merges naturally with the non-pertrubative estimate of the S-parameter close to the lower end of the conformal window obtained using gauge duality [arXiv:1007.0254 [hep-ph]]. Our results are natural benchmarks for lattice computations of the S-parameter for vector-like gauge theories and together with the lower bound constitute important constraints on models of dynamical electroweak symmetry breaking and unparticle physics.